Compounds of maleimide type are commonly prepared in two steps. The first step includes preparation of N-substituted maleamic acid by the reaction of a primary amine with maleic anhydride. The second step is cyclodehydration of the corresponding maleamic acid to maleimide derivative.
Preparation of maleamic acid derivatives is very simple and the yields are usually nearly quantitative. It is recommended to add the corresponding primary amine gradually into the solution of maleic anhydride, which is present in the solution in equimolar ratio or in excess with respect to amine. In this way possible addition of amine on a double bond of the corresponding maleamic acid is prevented.
Cyclodehydration of maleamic acid derivatives can be performed in several ways, for example by using chemical dehydration agents like acetic acid anhydride in the presence of sodium acetate. This method is preferred in preparation of numerous aromatic maleimides at laboratory conditions (U.S. Pat. No. 2,444,536). A disadvantage of this method is the fact that industrial application is economically disadvantageous because of a great amount of acid waste waters. Moreover, there are high demands on production facilities considering the corrosive effects of acetic acid.
Direct thermal cyclodehydration of maleamic acid derivatives can be performed at a temperature of near to 200° C. This method is impractical because of polymerization of the resulting maleimide derivative at extreme conditions. Thermal cyclodehydration may be performed at lower temperatures under the conditions of azeotropic distillation in the presence of acid catalysts. The use of an azeotropic solvent facilitates effective reaction water removing from the reaction system, thus moving the reaction equilibrium in favour of the required maleimide.
Suitable azeotropic solvents are cyclohexane, benzene, ethylbenzene, xylene isomers, cumene, chlorobenzene, buthylbenzene, diethylbenzene, mesitylene and the like.
Also boiling temperature of azeotropic solvents affects the reaction rate. The use of solvents with boiling point higher than that of toluene results in reducing the reaction time, but increasing the boiling point by solvent selection may result in an increase of the amount of by-products. Whereas the use of toluene as an azeotropic dehydrating agent results in low yield and long reaction time because of low solubility of maleamic acid in the reaction medium. These disadvantages are eliminated by adding polar aprotic solvents to the reaction mixture. In the patent literature, there are claimed many polar aprotic solvents, including dimethylformamide, dimethylacetamide, acetonitrile, N-methylpyrrolidone, dimethylsulfoxide and sulfolane (U.S. Pat. No. 5,484,948, U.S. Pat. No. 5,371,236). Dimethylformamide is the most often used auxiliary solvent. A disadvantage of DMF is its unlimited miscibility with azeotropic solvent, and as a consequence, its presence in the reaction mixture complicates processing the product. Polar aprotic solvent can be removed by washing it with water, but then waste waters containing DMF arise. A further disadvantage of using DMF and dimethylacetamide is the fact, that at the reaction conditions they hydrolyze and partially decompose.
The disadvantages of polar aprotic solvents are eliminated by quaternary ammonium salts (U.S. Pat. No. 4,225,498 U.S. Pat. No. 5,973,166 JP-54-30155), which are stable and, at suitably selected temperatures, they are immiscible with azeotropic solvents and, therefore, they can be easily removed from the reaction medium. In this method, the ratio quaternary ammonium salt/acid must be kept at a certain catalytic activity. The recycled catalyst must be additionally purified and adjusted to the required ratio, thus requiring increased financial demands on the process.